The invention relates to a pneumatically reversible ram boring device, in particular for trenchless preparation of ground bores and for trenchless installation of lines into the earth.
Ram boring devices of this type are also used for upsizing ground bores and for destructive replacement of underground pipes; they include a self-controlling percussion piston which automatically moves back and forth in a casing by means of compressed air and which transfers its kinetic energy to the casing in forward travel at a front dead center and in backward travel at a rear dead center.
Switchover from forward travel to backward travel is accomplished with the assistance of a reversing system which with the aid of operational air decelerates the percussion piston at its forward dead center essentially without transferring kinetic energy to the casing, and also moves the rear dead center backwards, thereby ensuring that the percussion piston transfers its kinetic energy at that point to the casing.
Several versions of such reversing systems are known. They employ for adjusting the two dead center positions a control sleeve which is disposed on a control pipe for axial movement by means of compressed air, on one hand, and oftentimes by means of a pre-tensioned compression spring, on the other hand, whereby the control pipe is arranged immobile in the device casing. The reversal is implemented essentially by way of two lines of development which differ in principle from one another by using different sources for the compressed air applied to the control sleeve during switchover.
For example, British Patent 1 540 344, which corresponds to U.S. Pat. No. 4,708,211, discloses a reversal in which the operational air (compressed air) is supplied via a compressed-air hose and a non-rotatable and immovable control pipe to the percussion piston for longitudinal movement of a control sleeve, when switching from forward travel to backward travel. Switchover from backward travel to forward travel is accomplished by interrupting the supply of operational air to the device, allowing the spring-biased control sleeve to move from its rear position to the forward position.
German Patent DE 39 09 567 C2, and the corresponding U.S. Pat. No. 5,148,878 discloses a spring-pneumatic reversal for reversing from forward travel to backward travel, using a spring-biased control sleeve, which is arranged in a guide sleeve for limited rotation by means of the compressed-air hose, for releasing a control sleeve under spring tension. This spring tension is dimensioned such that the control sleeve overcomes the spring bias when the operational air is fully applied, thereby moving the control sleeve from its forward position for the forward travel to its reverse position for the backward travel. To switch from backward travel to forward travel, the pressure of the operational air opposing the compression spring needs to be reduced to allow the compression spring to move the control sleeve to its forward position. In both positions, the control sleeve is rotated about its longitudinal axis by elastic means and thereby locked in place in the axial direction.
A drawback of these spring-pneumatic reversing systems is the particularly complex and expensive construction and the use of operational air from a single source for both the operation of the percussion piston and also for reversing the direction so that the operational air has to be switched off, causing an interruption of the operation of the device; or only a reduced device power is available in the event the pressure is reduced during reversing operation.
To enable reversal of the control sleeve independent of the operational air, ram boring devices are also known which use air from a separate compressed-air source to control the axial movement of a spring-biased control sleeve. Such a device is described in the German Patent DE 198 58 519 C2, which corresponds to U.S. Pat. No. 6,371,220.